Haemophilia A is an X-linked bleeding disorder which is characterized by the functional absence of blood coagulation factor VIII. Depending on the residual factor VIII activity in the plasma of the patient, haemophilia A can be classified as severe (factor VIII<1%), moderate (factor VIII 1-5%) or mild (>5%). Bleeding episodes in patients with haemophilia A can be effectively controlled by intravenous administration of purified factor VIII concentrates. These factor VIII-concentrates may be derived from pools of human plasma. Alternatively, recombinant factor VIII produced by geneticaly engineered eukaryotic cells may be used as a starting material for the preparation of factor VIII concentrates.
A serious complication of current haemophilia A treatment constitutes the development of neutralizing antibodies directed against factor VIII. These antibodies, commonly termed factor VIII inhibitors, arise in approximately 25% of the patients with severe haemophilia A, usually after 5-20 exposure-days (Ehrenforth et al. 1992, Lancet 339: 594-598). In patients with moderate and mild haemophilia A, anti-factor VIII antibodies occur less frequently and this is most likely due to induction of tolerance by endogenous factor VIII present in the plasma of this group of patients (McMillan et al. 1988, Blood 71: 344-348). Antibodies to factor VIII may develop with low frequency in healthy individuals.
Diagnosis of factor VIII inhibitors is commonly performed using the so-called Bethesda assay (Kasper et al. 1975, Thromb. Diath. Haemorrh. 34: 869-872). In this assay equal amounts of normal plasma and dilutions of inhibitor plasma are incubated for two hours at 37° C. Next, residual factor VIII activity is determined and compared to control incubation in which normal plasma is incubated with 0.1 M imidazole for 2 hours at 37° C. The amount of inhibitor is expressed in Bethesda units; one Bethesda unit corresponds to the amount of inhibitor that is capable of reducing the activity of factor VIII in normal plasma by 50%. A recent study has proposed several adaptations to the original assay system which serve to improve the stability of factor VIII during the assay (Verbruggen et al. 1995, Thromb. Haemostas. 73: 247-251). This so-called “Nijmegen modification” of the Bethesda assay is particularly useful for the detection of low titre factor VIII inhibitors. It should be noted that the Bethesda assay does not provide information on the epitopes of factor VIII inhibitory antibodies.
The occurrence of factor VIII-inhibiting antibodies renders factor VIII replacement therapy inadequate. Several treatment options are available to the clinician. Low titre inhibitors (up to 5-10 BU/ml) are usually treated with infusion of high doses of factor VIII. A subset of factor VIII inhibitors does not cross react with porcine factor VIII. Porcine factor VIII has been used for management of patients with an inhibitor. Administration of porcine factor VIII may present with side effects. After multiple treatment 30-50% of the patients develop antibodies that neutralize the activity of the administered porcine factor VIII.
An alternative treatment of patients with factor VIII inhibitor constitutes the use of factor VIII bypassing agents. Activated prothrombin concentrate complexes (APCC) have been used to bypass the activity of factor VIII. APCC has been used successfully to control bleeding episodes in a large number of patients with an inhibitor. However, treatment is not effective in all cases and an anamnestic rise in the titre of the inhibitor following administration of APCC (most likely due to trace amounts of factor VIII in the preparation) has been reported in a number of patients. In the last 5 years recombinant factor VIIa has become available as a new factor VIII bypassing agent for the treatment of patients with an inhibitor (Lusher et al. 1996. Haemostasis 26 (suppl. 1): 124-130). Recombinant factor VIIa has been successfully used to control the bleeding episodes in patients with an inhibitor. Treatment by this agent is however limited by the short half-life of this compound in the circulation which requires multiple infusions at relatively short time intervals. APC-resistant factor V has recently been suggested as an alternative means to bypass the biological activity of factor VIII inhibitors (WO 95/29259). The agents described above do not act directly on factor VIII inhibitors but merely serve to bypass factor VIII by infusion of large amounts of clotting factor concentrates with increased procoagulant activity.
Other methods of inhibitor neutralization have been proposed but their effectiveness has not been convincingly shown. Immunoglobulin preparations derived from plasma of healthy donors has been proposed as an active suppressor of factor VIII inhibitors (Sultan et al. 1984, Lancet 333, 765–768). Despite success in patients with acquired haemophilia A and high titre inhibitors, immunoglobulin preparations are not applied universally for treatment of patients with an inhibitor. The beneficial effects of immunoglobulin preparations in these patients have been attributed to the presence of anti-idiotypic antibodies that neutralize the activity of factor VIII inhibitors. Indeed in some patients the decline in the level of factor VIII inhibitors coincided with the appearance of anti-idiotypic antibodies (Sultan et al. 1987, Proc. Natl. Acad. Sci. USA 84: 828-831). Control of factor VIII inhibitors by anti-idiotypic antibodies in both haemophilia A patients and healthy individuals has been strongly advocated by some investigators (Gilles et al. 1996, J. Clin. Inv. 97: 1382-1388). The same group has proposed that infusion of antigen-antibody complexes in patients with inhibitors may accelerate a decline in anti-factor VIII antibodies in patients with an inhibitor (U.S. Pat. No. 5, 543,145). It has been proposed that this decline is mediated by an increase in the number of anti-idiotypic antibodies which are induced by the infused antigen-antibody complexes. The factor VIII specific antibody used in this treatment protocol is derived from plasma of patients with an inhibitor. Obviously, this presents a heterogeneous mixture of antibodies and no details with respect to the epitope specificity of these antibodies are available. Also the primary structure of the antibodies in these antigen-antibody preparations has not been disclosed.